In the GPS system, the signal emitted by a satellite is coded and the time taken by the signal to reach the point to be located is used to determine the distance between this satellite and this point, preferably called the pseudo-distance so as to take account of synchronization errors between the clock of the satellite and that of the station. These synchronization errors are conventionally eliminated by calculation when the signals are received from at least four different satellites. The determination of the distance between the point to be located and several satellites makes it possible, knowing the geographical coordinates of the satellites, to calculate the coordinates of the point to be located, usually coordinates expressed as latitude, longitude and altitude in a fixed terrestrial reference frame.
To determine the precise position of a mobile (accuracy of from a centimeter to a meter, as the case may be), a so-called “differential GPS” procedure is used, which consists in using, at the level of the mobile, for the calculation of its position, the errors noted with regard to each pseudo-distance at the level of a so-called reference station of known position.
This procedure makes it possible to correct the position calculation errors due in particular to trajectory deformations and to propagation. These errors are corrected, for example, by comparing for the reference station its known position and its calculated position arising from the measurement of the propagation time between the satellite and the reference station.